Silicones (i.e., organosiloxanes) are polymers containing alternating silicon and oxygen atoms in the backbone with various organic groups attached to the silicon atoms. Silalkylenesiloxane copolymers include alkylene backbone units without unsaturation and also include monovalent hydrocarbon groups attached to silicone atoms. Both silicones and silalkylenesiloxanes are useful materials for a wide variety of applications (e.g., rubbers, adhesives, sealing agents, release coatings, antifoam agents). Because of their biocompatibility, silicones present a low risk of unfavorable biological reactions and have therefore gained the medical industry""s recognition. Such materials are useful in a wide variety of medical devices. There are, however, limited materials available for medical device applications. In addition, there is a need for improved silicone materials that can be used in the medical industry, particularly those with good strength and tear resistance.
Prior to the present invention, silalkylenesiloxane copolymers have been prepared by three methods. Ring opening polymerization of cyclic silethylenesiloxane is disclosed in U.S. Pat. No. 5,117,025 (Takago et al.). Condensation polymerization of silanol terminated silalkylene oligomers is disclosed in U.S. Pat. No. 5,386,049 (Kishita et al.). Step growth hydrosilylation polymerization between a hydride terminated organosiloxane and an unsaturated aliphatic hydrocarbon that contains 2 carbon-carbon double bonds or one carbon-carbon double bond and one carbon-carbon triple bond is disclosed in U.S. Pat. No. 5,442,083 (Kobayashi).
U.S. Pat. No. 5,442,083 (Kobayashi) states that the ring opening polymerization of cyclic silethylenesiloxane is not advantagous for producing silalkylenesiloxane copolymers. As reported in Andrianov et al., Inst. Of Heteroorganic Cpds., p. 661, translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 739-44 (1971), a partial depolymerization occurs in this method, which therefore leads to reduced yields of the silalkylenesiloxane copolymer.
Step growth condensation polymerization of silanol terminated fluids yields copolymers that have silanol end groups. To make the copolymer end-functional, for example, vinyidimethylsilyl terminated, another synthetic step is required. This is a disadvantage. In addition, degrees of polymerization (Dp) from step growth silanol condensation polymerizations of disilanolsilalkylene compounds have been reported to be no greater than 180. See, U.S. Pat. No. 5,386,049 (Kishita et al.) and Benouargha et al., Eur. Polym. J., 33, p. 1117 (1997). This is a disadvantage.
Hydrosilylation step growth polymerization as a method of silalkylenesiloxane copolymer synthesis also contains inherent disadvantages. In order to produce high Dp copolymer, the stoichiometry of the silylhydride and unsaturated hydrocarbon moieties must be as close to 1:1 as possible. Side reactions which disturb this balance limit the Dp of said copolymer by creating terminating groups on unsaturated hydrocarbon monomers. For example, it is known in the art that transition metal catalysts typically used for hydrosilylation reactions can cause the isomerization of a terminal carbon-carbon double bond to an internal position. See, Harrod et al., Organic Synthesis via Metal Carbonyls, 2, John Wiley and Sons, New York, p. 673 (1977), Cundy et al., Adv. Organometallic Chem., 2, p. 253 (1973), and Speier, Adv. Organometallic Chem., 17, p. 407 (1979). This is a disadvantage. This isomerization renders the monomers less suceptible to hydrosilylation.
Silalkylenesiloxane copolymers having a Dp as high as 10,000 are disclosed in U.S. Pat. No. 5,484,868 (Kobayashi). However, step growth hydrosilylation polymerization was the method used to produce the copolymers and no examples were provided which would circumvent the disadvantages outlined above.
The following lists of documents disclose information regarding siloxane compounds.
All patents, patent applications, and publications listed above are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the application, certain of the information disclosed in the above-listed documents may be utilized in the monomers, polymers, their preparation methods, and the devices disclosed and claimed herein.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions and advantages to one or more of the problems existing in the prior art with respect to the preparation and properties of siloxanes, and particularly, silalkylenesiloxanes. Certain of these problems are discussed above. The materials and methods of the present invention address one or more of these problems.
The present invention provides symmetric cyclic silalkylenesiloxane monomers having the following formula: 
wherein p is at least 6, and v is at least 1, and each R1 and R2 group is independently a monovalent organic group. Alternatively, the present invention provides such monomers wherein p is at least 2 and v is at least 2. Such symmetric monomers include the same value for p in each repeat unit.
The present invention also provides asymmetric cyclic silalkylenesiloxane monomers having the following formula: 
wherein p and q are each at least 2, v and w are each at least 1, with the proviso that q does not equal p for at least one set of silalkylenesiloxane repeat units, and each R1 and R2 group is independently a monovalent organic group. Such asymmetric monomers can include values for p that are the same or different in the various repeat units, and values for q that are the same or different in the various repeat units.
The present invention also provides silalkylenesiloxane copolymers of the formula: 
wherein p is at least 2, b is at least 1, the sum of b and c is greater than 300, and each R group is independently a monovalent organic group. These copolymers can be crosslinked and/or reinforced with a silica filler. Preferably, they are both crosslinked and reinforced.
In one embodiment, the reinforced crosslinked material of the present invention is preparable from a silica filler and a crosslinked silalkylenesiloxane copolymer of the formula: 
wherein p is at least 6, b is at least 1, c is zero or greater, and each R group is independently a monovalent organic group.
The present invention also provides a medical device comprising a silalkylenesiloxane copolymer of the formula: 
wherein p is at least 2, b is at least 1, c is zero or greater, and each R group is independently a monovalent organic group. Preferably, this material is crosslinked, and more preferably, it is crosslinked and compounded with a silica filler.
The present invention also provides methods of making silalkylenesiloxane copolymers of the present invention. In one embodiment, a method involves combining at least one cyclic silalkylenesiloxane monomer with a catalyst. In another embodiment, a method involves combining at least one compound having at least one silalkylenesiloxane unit and at least one compound having at least one siloxane unit with a catalyst. Preferably, the starting compounds (silalkylenesiloxanes and siloxanes) can be linear or cyclic and the catalyst can be acidic or basic.
Herein, the values for the variables in the formulas are integers; however, they can be average values if the formulas represent average structures, such as occurs with polymers.
As used herein, the term xe2x80x9ccopolymerxe2x80x9d refers to polymers having two or more different repeat units and includes copolymers, terpolymers, tetrapolymers, etc.
As used herein, the term xe2x80x9corganic groupxe2x80x9d means a hydrocarbon group that is classified as an aliphatic group, cyclic group, or combination of aliphatic and cyclic groups (e.g., alkaryl and aralkyl groups). In the context of the present invention, the term xe2x80x9caliphatic groupxe2x80x9d means a saturated or unsaturated linear or branched hydrocarbon group. This term is used to encompass alkyl, alkenyl, and alkynyl groups, for example. The term xe2x80x9calkyl groupxe2x80x9d means a saturated linear or branched hydrocarbon group including, for example, methyl, ethyl, isopropyl, t-butyl, heptyl, dodecyl, octadecyl, amyl, 2-ethylhexyl, and the like. The term xe2x80x9calkenyl groupxe2x80x9d means an unsaturated, linear or branched hydrocarbon group with one or more carbon-carbon double bonds, such as a vinyl group. The term xe2x80x9calkynyl groupxe2x80x9d means an unsaturated, linear or branched hydrocarbon group with one or more carbon-carbon triple bonds. The term xe2x80x9ccyclic groupxe2x80x9d means a closed ring hydrocarbon group that is classified as an alicyclic group, aromatic group, or heterocyclic group. The term xe2x80x9ccyclic groupxe2x80x9d means a cyclic hydrocarbon group having properties resembling those of aliphatic groups. The term xe2x80x9caromatic groupxe2x80x9d or xe2x80x9caryl groupxe2x80x9d means a mono- or polynuclear aromatic hydrocarbon group. The term xe2x80x9cheterocyclic groupxe2x80x9d means a closed ring hydrocarbon in which one or more of the atoms in the ring is an element other than carbon (e.g., nitrogen, oxygen, sulfur, etc.).
As is well understood in this technical area, a large degree of substitution is not only tolerated, but is often advisable. Substitution is anticipated on the compounds of the present invention. As a means of simplifying the discussion and recitation of certain terminology used throughout this application, the terms xe2x80x9cgroupxe2x80x9d and xe2x80x9cmoietyxe2x80x9d are used to differentiate between chemical species that allow for substitution or that may be substituted and those that do not allow or may not be so substituted. Thus, when the term xe2x80x9cgroupxe2x80x9d is used to describe a chemical substituent, the described chemical material includes the unsubstituted group and that group with O, N, or S atoms, for example, in the chain as well as carbonyl groups or other conventional substitution. Where the term xe2x80x9cmoietyxe2x80x9d is used to describe a chemical compound or substituent, only an unsubstituted chemical material is intended to be included. For example, the phrase xe2x80x9calkyl groupxe2x80x9d is intended to include not only pure open chain saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, t-butyl, and the like, but also alkyl substituents bearing further substituents known in the art, such as hydroxy, alkoxy, alkylsulfonyl, halogen atoms, cyano, nitro, amino, carboxyl, etc. Thus, xe2x80x9calkyl groupxe2x80x9d includes ether groups, haloalkyls, nitroalkyls, carboxyalkyls, hydroxyalkyls, sulfoalkyls, etc. On the other hand, the phrase xe2x80x9calkyl moietyxe2x80x9d is limited to the inclusion of only pure open chain saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, t-butyl, and the like.
The present invention provides cyclic silalkylenesiloxane monomers that can be preferably polymerized using ring opening polymerization either alone or in the presence of siloxane monomers to yield silalkylenesiloxane random and block copolymers. This methodology facilitates high degrees of polymerization since the cyclic silalkylenesiloxane monomers can be easily purified and the ring opening polymerization is efficient. Alternatively, the polymers of the present invention can be prepared by coequilibrating mixtures of cyclic and linear species.
The copolymerization reactions preferably utilize similar chemistry as that known in the art for silicone materials to yield copolymers having various functionality pendant and/or terminal to the polymer backbone. Pendant and/or terminally functional silalkylenesiloxane copolymers are believed to be useful as elastomers, adhesives, and sealing agents. Such copolymers are capable of being crosslinked. The crosslinked materials are believed to be suitable for a variety of applications, including, elastomers, adhesives, sealing agents, and the like. They are believed to be particularly suitable for use in medical devices.
As used herein, medical device refers to a device that has surfaces that contact tissue, blood, or other bodily fluids in the course of their operation, which fluids are subsequently used in patients. This can include, for example, extracorporeal devices for use in surgery such as blood oxygenators, blood pumps, blood sensors, tubing used to carry blood and the like which contact blood which is then returned to the patient. This can also include endoprostheses implanted in blood contact in a human or animal body such as vascular grafts, stents, pacemaker leads, heart valves, and the like that are implanted in blood vessels or in the heart. This can also include devices for temporary intravascular use such as catheters, guide wires, and the like which are placed into the blood vessels or the heart for purposes of monitoring or repair.
Silalkylenesiloxane cyclic monomers are made by first preparing a silalkylenesiloxane copolymer via hydrosilylation, for example. These copolymers are generally of a relatively low molecular weight (e.g., having a degree of polymerization (Dp) of no greater than about 200) and are not functionalized. Depolymerizing said polymer to cyclic monomers, then purifying by distillation or recrystallization affords monomers that can be polymerized in the presence of linear and/or cyclosiloxane monomers. By this method, a variety of functionality can be incorporated pendant or terminal to the silalkylenesiloxane copolymers. Also, higher molecular weight silalkylenesiloxane copolymers (e.g., having a Dp of greater than about 300) can be prepared.
Specifically, alternating silalkylenesiloxane copolymer starting materials can be prepared by polymerizing a dihydrodisiloxane with an xcex1,xcfx89-alkadiene, as represented by the following scheme: 
wherein s is at least 2, and preferably no greater than 26, and n is at least 1, and preferably no greater than 200. Each R1 and R2 group is independently (i.e., they may be the same or different) a monovalent organic group (preferably, a C1-C30 organic group). Preferably, R1 and R2 are independently methyl, ethyl, propyl, or other alkyl group; phenyl, tolyl, xylyl, or other aryl group; benzyl, phenethyl, or other aralkyl group. These groups may be substituted in part or in total (i.e., such that all the hydrogen atoms are replaced) with various groups, such as halogen atoms. More preferably, R1 and R2are C1-C4 alkyl groups, and most preferably, methyl moieties.
Alternating silalkylenesiloxane copolymer starting materials may also be prepared by polymerizing a dihydrodisiloxane with an xcex1,xcfx89-alkadienedisiloxane, as represented by the following scheme: 
wherein R1, R2, and n are as described above, t is zero or greater, and preferably, zero to 6, most preferably zero to 1.
Transition metal compounds that catalyze hydrosilylation reactions may be used to catalyze the above reactions. Preferred catalysts include, but are not limited to, platinum 1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex and platinum 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetra-siloxane complex. The addition polymerization is preferably carried out at a temperature slightly under the reflux temperature of the reactants in the absence of any solvent. During the course of the reaction, the temperature may be raised since the intermediate silalkylenesiloxane structures (e.g., dimer, trimer, tetramer, etc.) have a higher boiling point than the starting reactants. The ratio of reactants is typically 1:1 to afford polymer.
Generally, the silalkylenesiloxane copolymer starting materials are of the following formula: 
wherein R1 and R2 are as described above, p is at least 2, and n is at least 1. Preferably, p is at least 6 and n is at least 100. More preferably, p is no greater than 30 and n is no greater than 300. Such materials are suitable for preparing cyclic monomers as described below. The repeat unit in formula 1 is referred to herein as a silalkylenesiloxane unit, wherein xe2x80x9calkylenexe2x80x9d refers to the hydrocarbon chain in the backbone.
In addition, asymmetric silalkylenesiloxane copolymer starting materials can be produced when mixtures of xcex1,xcfx89-alkadienes and/or xcex1,xcfx89-alkadienedisiloxanes are used in schemes 1 or 2 above. Such silalkylenesiloxane copolymers are of the following general formula: 
wherein R1, R2, p, and n are as described above, q is at least 2 and m is at least 1, with the proviso that q does not equal p for at least one set of silalkylenesiloxane repeat units. Preferably, q is at least 6 and m is at least 100. More preferably, q is no greater than 30 and m is no greater than 300. Such materials can include various asymmetric polymers wherein the values for p can be the same or different throughout the various repeat units, and the values for q can be the same or different throughout the various repeat units. Such materials are suitable for preparing cyclic monomers as described below.
The silalkylenesiloxane copolymer starting materials as shown above can be depolymerized under vacuum with heat and a base to produce mixtures of cyclic monomers. Either an alkali metal hydroxide such as potassium hydroxide or alkali metal silanolate such as potassium trimethylsilanoate can be used to perform the copolymer cracking. Sufficient temperature and vacuum are used in order to remove cyclic silalkylenesiloxane monomers by distillation so as to continually drive the depolymerization equilibrium. Pot temperatures as high as 300xc2x0 C. and vacuum as low as 20 mTorr are typically used to distill the large cyclic rings away from the copolymer.
The copolymers described above can be depolymerized without removal of the platinum or transition metal catalyst. However, under the conditions of the depolymerization reaction, such catalysts can promote degradation of the polymer, which leads to copolymer crosslinking. This, therefore, reduces the yield of cyclic monomer produced. Higher yields are obtainable when the residual catalyst is removed.
Such symmetric cyclic silalkylenesiloxane monomers of the present invention are of the following general formula: 
wherein R1, R2, and p are as defined above, v is at least one. Preferably, v is no greater than 3. For certain preferred embodiments, p is at least 2 when v is at least 2.
Silalkylenesiloxane monomers of the present invention are not limited to symmetrical structures. When a mixture of xcex1,xcfx89-alkadienes having different carbon lengths are used to prepare the silalkylenesiloxane copolymer via hydrosilylation, for example, a copolymer as previously described in formula 2, can be depolymerized to a mixture of asymmetric compounds. Such asymmetric silalkylenesiloxane monomers of the present invention are of the following general formula: 
wherein R1, R2, p, q, and v are defined as above, w is at least 1. Preferably, w is no greater than 3. More preferably, the sum of v and w is no greater than 3. Such materials can include various asymmetric cyclic monomers wherein the values for p can be the same or different in the various repeat units, and the values for q can be the same or different in the various repeat units. Again, q does not equal p for at least one set of silalkylenesiloxane repeat units.
The cyclic silalkylenesiloxane monomers can be purified by distillation and/or recrystallization from polar solvents, such as ethanol, for example. Preferably, they are purified before preparing the silalkylenesiloxane copolymers of the present invention as described below.
Cyclic silalkylenesiloxane monomers can be polymerized using methods that are similar to those used for cyclic siloxanes. For example, depending upon the ring size, the cyclic silalkylenesiloxane monomers can undergo ring opening reactions under either anionic and cationic catalysis. The anionic polymerization of cyclic silalkylenesiloxane monomers can be initiated by alkali metal oxides and hydroxides, silanolates and other bases, preferably, potassium hydroxide and potassium trimethylsilanoate. Alternatively, cationic polymerization can be initiated by protonic and Lewis acids, preferably triflic acid and strongly acidic ion-exchange resins.
Typically, both anionic and cationic ring opening polymerizations (ROP) may be performed without the use of solvents. However, in order to deliver well-controlled amounts of catalyst to reaction mixtures, solvents such as toluene may be used to dilute said catalyst. Both the anionic and cationic catalyzed equilibration reaction conditions (e.g., time and temperature) are similar to those known in the art for ROP of cyclic organosiloxanes. For example, the triflic acid catalyzed ROP of cyclic silalkylenesiloxane monomers typically requires a catalyst concentration of about [7xc3x9710xe2x88x924] to about [5xc3x9710xe2x88x923] and, once added to the cyclic monomer mixture, the equilibration reaction is complete within about 30 minutes to several hours.
The ability to prepare highly pure cyclic silalkylenesiloxane monomers enables the invention herein to produce much higher degrees of polymerization (e.g., Dp values of greater than about 300) than step growth silalkylenesiloxane copolymers previously know in the art. Such silalkylenesiloxane copolymers contain the same structure as defined in formula 1, with the proviso that n is preferably greater than 300. Preferably, n is no greater than 10,000.
The utility of the present invention is particularly appreciated when cyclic silalkylenesiloxane monomers (including mixtures of symmetric and asymmetric cyclic monomers) are copolymerized in the presence of cyclic and/or linear siloxane compounds. A representative synthesis of such copolymers are described, for example, by scheme 3 as follows: 
wherein R1, R2, p, and v are as defined above. The value of x is at least 3 and preferably, no greater than 3. The value of c can be zero or greater, although, preferably, it is at least 1, more preferably, at least 3, and most preferably, at least 50. The value of b is at least 1, preferably, 5 at least 50, although more preferably, b is at least equal to C. Preferably, the sum of b and c is no greater than 10,000. Each R3 and R4 group is independently a monovalent organic group (preferably, a C1-C30 organic group). Preferably, R3 and R4 are independently methyl, ethyl, propyl, butyl, pentyl, hexyl, or other alkyl group; vinyl or other alkenyl group; phenyl, tolyl, xylyl, or other aryl group; benzyl, phenethyl, or other aralkyl group. These groups may be substituted in part or in total (i.e., such that all the hydrogen atoms are replaced) with various groups, such as halogen atoms, cyano groups, and amino groups. More preferably, R3 and R4 are methyl, phenyl, and vinyl moieties. The resultant copolymers can be random or block copolymers with the value of p being the same or different in the repeat units. Herein, the structural unit containing R3 and R4 groups in the above scheme is referred to as a siloxane unit and the structural unit containing the R1 and R2 groups is referred to as a silalkylenesiloxane unit.
Furthermore, silalkylenesiloxane copolymers containing terminal and/or pendant functional groups can be produced, for example, as shown in scheme 4: 
wherein R1, R2, R3, R4, p, v, x, b, and c are as defined above, each R5 group is independently a monovalent organic group (preferably, a C1-C30 organic group). Preferably, each R5 is independently methyl, ethyl, propyl, butyl, pentyl, hexyl, or other alkyl group; vinyl, allyl, or other alkenyl group; phenyl, tolyl, xylyl, or other aryl group; benzyl, phenethyl, or other aralkyl group. These groups may be substituted in part or in total (i.e., such that all the hydrogen atoms are replaced) with various groups, such as halogen at oms, cyano groups, amino groups. More preferably, each terminal silyl group includes at least one R5 which is a vinyl moiety. The resultant copolymers can be random or block copolymers with the value of p being the same or different in the repeat units.
Interestingly, the alternating silalkylenesiloxane copolymers containing the structures described by formulas 1 and 2, when crosslinked, do not exhibit reinforcement by treated fumed silica. However, reinforcement and enhanced physical properties are obtained when treated fumed silica is compounded with copolymers containing a siloxane block length of greater than 2 (e.g., wherein c is equal to or greater than b in scheme 4). These preferred functionalized copolymers can be compounded with a silica filler (e.g., fumed silica) and/or crosslinked using similar chemistry as know in the art for silicone rubber.
Preferably, silalkylenesiloxane copolymers of the present invention have the following general formula: 
wherein R1, R2, R3, R4, R5, p, b, and c are as defined above. As used herein, this formula represents both random and block copolymers. Such materials can include symmetric polymers and various asymmetric polymers wherein the values for p can be the same or different in the various repeat units.
For certain embodiments of the copolymers in the above formula, p is at least 2 when b is at least 1 and the sum of b and c (which can be referred to as the degree of polymerization or Dp) is greater than 300. For other embodiments of the copolymers, p is at least 2, preferably, at least 6, when b is at least 1 and c is zero or greater.
For certain embodiments of the copolymers in the above formula, one or more of the R groups (R1, R2, R3, R4, and/or R5) include crosslinkable functionalities, such as vinyl, alkoxy, acetoxy, enoxy, oxime, amino, hydroxyl, cyano, halo, acrylate, epoxide, isocyanato groups, etc. For particularly preferred embodiments, copolymers, whether crosslinked or not, are compounded with a silica filler, which typically provides reinforcement and better physical properties for certain applications. For such materials, the sum of b and c (Dp) is preferably 1000 to 5000.
Cyclic silalkylenesiloxane monomers can be polymerized using methods that are similar to those used for cyclic siloxanes, as described above. Alternatively, the above silalkylenesiloxane copolymers may be prepared by coequilibrating mixtures of cyclic and/or linear species. Coequilibrations can be performed under the same anionic or cationic reaction conditions as described above for ROP of silalkylenesiloxane copolymers. For example, a cyclic silalkylenesiloxane monomer as described in formula 3 can be equilibrated with a linear siloxane polymer to yield a silalkylenesiloxane copolymer. In addition, a cyclic siloxane monomer can be equilibrated with a silalkylenesiloxane copolymer to afford a silalkylenesiloxane copolymer having incorporated additional said siloxane units. Alternatively, a linear silalkylenesiloxane copolymer and linear siloxane polymer may be equilibrated together to afford a copolymer which contains a summation of both linear starting reagent units.
Thus, the present invention provides methods for the preparation of silalkylenesiloxane copolymers, which involve the use of cyclic silalkylenesiloxane monomers, particularly those described above. Preferably, the present invention provides a method that involves combining at least one compound having at least one silalkylenesiloxane unit and at least one compound having at least one siloxane unit with a catalyst. The compounds having at least one silalkylenesiloxane unit can be cyclic or linear, preferably, they are cyclic silalkylenesiloxane monomers. The compounds having at least one siloxane unit can be cyclic or linear, preferably, they are cyclic siloxane monomers. The catalysts can be acidic or basic compounds as described above. Thus, as discussed above, the reaction conditions can involve cationic or anionic polymerization.
In order to prepare crosslinked silalkylenesiloxane materials, it is preferred for the copolymers to be functionalized and miscible with the crosslinker. When the alkylene content of a silalkylenesiloxane copolymer is greater than about 15% by weight, the copolymer is not miscible with conventional polysiloxane crosslinking materials. However, if both crosslinking functionalities reside terminal and/or pendent to silalkylenesiloxane copolymers, the materials are typically miscible and will react. For example, the vinyldimethylsilyl terminated silalkylene prepared in Example 4.1 and used within the compounded rubber in Example 5.3 contains 10.6% alkylene by weight. This facilitated the use of a conventional poly(hydromethylsiloxane-co-dimethylsiloxane) crosslinker. However, the vinyidimethylsilyl terminated silalkylenesiloxane copolymers used in Examples 5.1 and 5.2 contained 39% and 29% alkylene by weight, respectively. These copolymers are not misible with conventional poly(hydromethylsiloxane-co-dimethylsiloxane) crosslinker. Therefore, a silalkylenesiloxane copolymer containing hydromethylsiloxane units and containing 30% by weight alkylene units was used to crosslink said materials.